Radio frequency (RF) communications systems incorporate antennas at the transmitter and receiver to enable efficient transfer of RF signals propagating through space from the transmitter to the receiver. The transmitted signal generally propagates in a uniform way such as in a straight line unless there are obstructions along the path, like building clutter, or other man made or natural obstructions. When a cluttered environment is present, any number of reflections and diffractions may occur as the signal interacts with the environment. A receive antenna may collect a large number of signals in such an environment, the first arriving signal having traveled the shortest distance, and later arriving signal having traveled additional distance due to the signal path reflecting off a building, or diffracting around a corner. When these numerous signals sum together at the receive antenna, variations in amplitude occur caused by the phases and amplitudes of the signals in combination. This variation in amplitude is called fading. In the wireless environment, Rayleigh or Rician fading is well known in the art.
In a communication system, diversity reception is usually desired to combat fading, but the limitations of subscriber units often makes it difficult or impossible to implement diversity within the constraints involved. In mobile and portable subscriber units with a single antenna, the effect of fading requires additional signal strength to be sent compared to what is required when diversity antennas are used to achieve the same level of performance. This extra signal strength reduces the effective coverage area of the subscriber unit. In an environment where the wireless terminal is fixed, the fading problem may be worse, since the fixed terminal is mounted to a building or otherwise fixed and if a fade occurs, the fade may last for a long time. This is typically not the case with mobile units or portable units, which move around and therefore move in and out of fades very quickly, even if travelling at slow speeds.
In order to combat fading, antenna diversity is often used. Antenna diversity typically incorporates two or more antennas physically separated in space to avoid fades or nulls on a given antenna or branch. This may be accomplished by a number of different diversity techniques which are well known in the art, such as: combining, selecting or switching. These diversity techniques allow the signals on the antennas that are not experiencing fades to be used in whole or in part, and the antennas receiving the signal that are in a fade to be used to a lesser extent, or not at all.
Diversity generally requires two separate and distinct antennas from which the best signal, and correspondingly the best antenna, is chosen by various known diversity methods. This implies two antenna elements, two RF cables, and an electronic switch in the simplest diversity technique. In many cases, the expense of providing a diversity function is too high, particularly in subscriber units, where space, parts count, and constraints due to the structure of the handset make it difficult to incorporate multiple antenna elements. Thus there is a need for an improved antenna apparatus that removes the limitations of the prior art.